Genetic trends can decrease IQ, and as long as we want the fruit of technological civilization, we have to care about that. There are three main ways in which IQ could decrease:
I. Selection could favor lower IQ within a group. How fast?
II. Demographic changes- groups with lower IQ could be immigrating, or differences in birth rates could mean that smarter groups are declining relative to other groups.
III. Relaxed selection. It looks as if a lot of the variance in IQ is due to rare deleterious variants generated by mutation. Over the long run, selection has eliminated those deleterious mutations as fast as they were generated (mutation-selection balance). Over the last few generations, selection has weakened: a smaller fraction of babies are dying. Even though most of those babies in the past were dying of disease or starvation, not directly connected to baby IQ, babies in better genetic shape were more likely to survive. Smarter people seem to live longer because they’re in better genetic shape – ultimately because a huge fraction of the genome is expressed in the brain and influences intelligence. Being smarter means that on average you’re in better genetic shape, while being in better genetic shape – having lower genetic load – means that on average you’ll be smarter.
Weaker selection -> increasing genetic load -> lower IQ. But how fast?
All three of these processes must be happening today in the United States. I and III are probably happening everywhere.